A power amplifier (PA) is one of the necessary devices in many systems and the distortion of the signals caused by nonlinear characteristics of power amplifier is one of the main reasons for the reduction of power amplifier power efficiency. Therefore, it is of importance to take linearization techniques to improve the power efficiency of PA. Among the numerous linearization techniques, digital pre-distortion techniques are widely used and fall into two main technical categories: look-up table (LUT) based and polynomial based technique. However, LUT based pre-distortion techniques require large random access memory storage and has a slow convergence speed. Without limiting the scope of the invention, its background is described in connection with systems, apparatus and methods for correcting the nonlinearity of a power amplifier of the type that may be used in ultrasonic applications.
For example, U.S. Pat. No. 8,064,851, entitled, “RF Transmitter with Bias-Signal-Induced Distortion Compensation and Method Therefore” discloses an RF transmitter generates non-DC bias signals configured to improved power-added efficiency (PAE) in the operation of an RF amplifier. The RF amplifier generates an amplified RF signal which, due to the addition of the bias signals, includes bias-signal-induced RF distortion. The bias signals drive a bias-induced distortion cancellation circuit that adjusts the bias signals to compensate for the influence of impedances experienced by the bias signals before being applied to the RF amplifier. After mixing with a baseband communication signal, adjusted bias signals are combined into a composite baseband signal, upconverted to RF in an upconversion section 84, and applied to the RF amplifier where they cancel at least a portion of the bias-signal-induced RF distortion.
Another example includes, U.S. Pat. No. 8,064,850, entitled, “High Efficiency Linearization Power Amplifier for Wireless Communication” discloses an embodiment of the invention uses a predistortion correction signal to combination the modulated RF signal by an analog multiplier for linearization of power amplifiers having nonlinear characteristics such as those used in wireless RF transmitters. A predistortion controller comprises a plurality of down converters for retrieving both the ideal non-distorted information and the feedback distorted information, together with pre-stored digitally-indexed predistortion information stored, for example, in a look-up table. The digitally-indexed information models nonlinear characteristics of the high power amplifier, and is stored prior to processing of pre-compensation in the power amplifier. When the predistortion information is combined with the modulated RF signal in the analog multiplier, the result is a substantially linear information transmission from the power amplifier. In an embodiment of the system, the modulated RF input signal and the feedback signal from PA output are down-converted, respectively, by analog devices, such as mixers, after which the analog intermediate frequency (IF) signals are digitized by analog-to-digital converters for digital predistortion correction processing, followed by predistortion processing performed by, for example, a DSP or FPGA chip to generate a digital correction control signal, which is then converted to an analog signal by a digital-to-analog converter, followed by combining the analog correction signal with the RF modulated input signal to yield the input to the power amplifier.
U.S. Pat. No. 7,847,631, entitled, “Method and Apparatus for Performing Predistortion” discloses embodiments described herein to provide better predistortion solutions for the linearization of high power amplifiers, especially those with memory effects. Also described are predistorter apparatus embodiments in which a predistorter includes complex multipliers for multiplying the value of each Pkq having the same value of k to produce K products and at least one adder for adding the K products to generate predistorted signal zn from input signal xn in accordance with the equation above. The entire contents of which are incorporated herein by reference.